Certain medical implants and orthopedic implants require strength for weight bearing purposes and porosity to encourage bone/tissue in-growth. For example, many orthopedic implants include porous sections that provide a scaffold structure to encourage hone in-growth during healing and a weight bearing section intended to render the patient ambulatory more quickly. Rapid manufacturing technologies (RMT), particularly direct metal fabrication (DMF) and solid free-form fabrication (SFF), have been used to produce metal foam used in medical implants or portions of medical implants. In general, RMT methods allow for structures to be built from 3-D CAD models, including tessellated/triangulated solids and smooth solids. For example, DMF techniques produce three-dimensional structures one layer at a time from a powder which is solidified by irradiating a layer of the powder with an energy source such as a laser or an electron beam. The powder is fused, melted or sintered, by the application of the energy source, which is directed in raster-scan fashion to selected portions of the powder layer. After fusing a pattern in one powder layer, an additional layer of powder is dispensed, and the process is repeated with fusion taking place between the layers, until the desired structure is complete.
While DMF can be used to provide dense structures strong enough to serve as weight bearing structures in medical implants, the porous structures conventionally used employ arrangements with uniform, non-random, and regular features that create weak areas where the struts of the three-dimensional porous structure intersect. Further, conventional porous structures with uniform, non-random, and regular features that do not resemble trabecular bone structures. The disclosures of International Application Nos. PCT/US2010/046022, PCT/US2010/046032, and PCT/US2010/056602 address these drawbacks by providing efficient methods to manufacture three dimensional porous structures and the structures themselves, with randomized scaffold structures that provide for improved porosity without sacrificing the strength, improved strength including seamless junctions between units, and improved connectivity, as well as trabecular features. The disclosures of International Application Nos. PCT/US2010/046022, PCT/US2010/046032, and PCT/US2010/056602 are incorporated herein by reference in their entirety.
The RMT build process, however, often does not precisely form the porous structure according to the design input. In particular, due to the directionality of the build process for rapid manufacturing machinery, objects built in the X-Y plane do not necessarily look the same if built in either the Y-Z or X-Z planes. The Z-dimension (or vertical direction) is more difficult to control when the build process plots in layers oriented in the X-Y plane. In other words, an aperture oriented in the X-Z plane that is supposed to have similar shape and size to another aperture in the X-Y plane takes on a slightly different shape and size when is built.
While this disparity is usually negligible in many cases, it is quite apparent when the built structure itself is quite small, such as structures intended for biologic ingrowth. Minor disparities caused by the RMT build process in the structures intended for biologic ingrowth can result in structures that are not optimal for its purpose. For instance, the structures can include struts that are predisposed to failure because they are thinner or more elongated than as dictated by specification or the structure may not have the optimal porosity because struts are thicker than specified, as well as the thinner or more elongated struts.
In light of the above, there is still a need for efficient methods to address directional disparities of structures built by the RMT process or other manufacturing processes that suffers from similar directional disparities, e.g., affected by relative proximity to a heat source/sink or layer-by-layer build approach, particularly structures intended for biologic ingrowth.